1. Field of the Invention
The present invention relates to a hot-melt type ink for use in a thermal ink-jet printer which projects a heat-melted ink-jet through a thermal head.
2. Prior Art
Several systems have been proposed for ink-jet recording: electric field controlled jet that is ejected under an electrostatic attractive force; drop-on-demand ink (impulse jet) that is ejected under the oscillating pressure created by a piezoelectric transducer; and thermal ink jet that is ejected under the pressure created by air bubbles formed and grown with heat. These and other known methods of ink-jet recording are capable of producing image of a very high resolution.
Inks used in ink-jet printers are generally of two types: water-based inks which employ water as the principal solvent, and organic solvent-based inks which employ organic solvents as the principal solvent. Images printed with water-based inks are usually low in water resistance whereas organic solvent-based inks are capable of providing printed images with improved water resistance.
However, these water- and organic solvent-based inks, which are liquid at room temperature, will be liable to blot the recording paper upon printing and are not usable to give a satisfactory printing density. Furthermore, the liquid nature of these inks causes high incidence of deposition of crystallizable matter from the inks and this has been a major cause of undesirably low reliability of the conventional ink-jet recording systems.
With a view to eliminate the defects of these prior art inks of the solution type, the use of hot-melt type inks that are solid at room temperature has been proposed as inks for thermal ink-jet printers. For example, U.S. Pat. No. 3,653,932 proposes an ink containing a dialkyl ester of sebacic acid; U.S. Pat. No. 3,715,219 discloses an ink containing a higher aliphatic alcohol; U.S. Pat. No. 4,390,369 shows an ink containing a natural wax; EP 99,682 proposes an ink containing stearic acid; and U.S. Pat. No. 4,659,383 discloses an ink that contains an acid or alcohol of C.sub.20 -C.sub.24, optionally in the presence of a ketone having a comparatively high melting point. However, not all of these inks exhibit high solubility of dyes and the types of dyes that can be used are limited. In addition, these inks have not necessarily been stable against prolonged heating or repeated heat cycles. EP 181,198 discloses a hot-melt type ink that has a solid pigment dispersed in a wax having a melting point higher than 65.degree. C. or an aliphatic acid or alcohol of C.sub.18 C.sub.24 This ink, however, has the problem of poor dispersion stability.